It was at her aunt's 90th birthday party in 2004 when Stella Kinrick discovered the truth about her father. "My aunt said she couldn't stand carrying the secret any longer and had I never wondered why I don't look remotely like my brother. We were, she explained, both born as a result of sperm donors, and our father was not in fact our father at all," explains Stella, 52.

Stunned, Stella confronted her mother. "My father had died when I was 22 and so I asked exactly when she had planned to tell me. Her answer was simple: never."

Friends tell Stella that having no information about her true paternity shouldn't make any difference to who she is, "especially at my age". "But it makes all the difference in the world," she insists. "I don't know anything about one whole side of me. I've been diagnosed with skin cancer, which I've been told is probably genetic, and I wonder what else I've inherited from him. Do I have half-siblings out there? Does my love of music and writing come from my father's side? Could I be Eastern European or even Jewish, since I know that many of the donors back then were Eastern European Jewish doctors? Some of these questions impact on my own children."

Most of us take it for granted who our father is, but for many people - whether they're donor-conceived, adopted, born as a result of a brief relationship or for a host of other reasons - there is scant, if any, information available. Worse still, there is little hope of finding any.

Kate Trench, a psychodynamic counsellor, many of whose clients lack knowledge about their father, says it's common for such people to feel unsettled and ill at ease with themselves, often for their entire lives. It all comes down to an unfulfilled sense of identity, she explains. "Our identities are multifaceted and are made up of ethnic identity, social identity, sexual identity and cultural identity, just to name a few. Not having a father can impact on every one of these things," she says.

This is often coupled with an unresolved sense of loss. People say you can't miss what you haven't had, but no group of people reveal this as being more misguided than people who long to know their paternity. "If it hasn't been worked through - possibly because there is still the hope of one day finding the father - this unmourned loss can occupy a huge inner space of a person's psyche," says Trench.

In many situations, people have not lacked a father figure in their lives, albeit not a genetic dad. Indeed, few people wanting to know more about their biological fathers say they are seeking a paternal relationship. They simply want some questions answered.

Like many people, Stella's anger and hurt stems not only from the "sealed" information but also the lies that surrounded it. "After 50 years, you're pretty OK with who you are. You've long since done the teenage bit of desperately trying to find your identity, but when that's pulled away from you very suddenly, you feel powerless and vulnerable. It's also led to a lot of problems with relationships because of issues around trust. It makes you think, who else might be lying?"

Dr Sharon Pettle, consultant clinical psychologist at Great Ormond Street hospital, has researched the effects of secrets about biological parentage. "When a family secret like this comes out - often from a source that nobody has anticipated - the feelings of shock, distrust and betrayal cannot be overestimated, no matter how carefully and sensitively the content is portrayed. One woman summed it up by saying that it was as if the entire scaffolding of her life had fallen down."

Children work hard to grasp a sense of identity from an early age. When they ask how they got into Mummy's tummy and how they got out again, it is part of the search for knowledge about themselves. Dr Pettle adds that there is a very strong emphasis in our culture about similarities in looks, mannerisms and behaviours between parents and their offspring, which serves to confirm the child's place within their kinship group and in the world. "Having that place swept aside can be a hugely powerful experience and I've known some adults to be severely affected psychologically, and experience a period where their emotional stability was severely compromised. They are left with a gaping hole."

Julie Holguin-Rodriguez, a 42-year-old adoptee, has been more fortunate. Having discovered that her birth mother had died, it became all the more pertinent for her to find her birth father. "In the years between finding out about my birth mother and finding him, people suggested I should just get on with my life and leave the past in the past, but I became more and more curious," she says. "When my son had his first birthday, I realised just how strong my yearning was to know more about my family background - for him as much as me."

Eight years after her reunion with her birth father, the significance of the relationship and the feeling of "completeness" still surprises her. "I'd tried to keep level-headed and accept the search might not be successful and that he might not want to meet me, and I certainly never dared consider a relationship. But we had an instant bond. We are very alike in looks and character. It feels really good to know him and my history. By the time I got married, my adoptive father had died and my birth father gave me away. That made me so happy."

Not everyone wants to know their biological father, however. Shaun Heard, a 39-year-old adoptee, did look for his natural parents in his 20s, but decided against it in the long run. "I was brought up by white parents, but I'm half-Pakistani and so my original motivation was wanting to find people who looked like me," he explains. "When I'd traced my birth mother, I wanted to know more about my birth father, particularly as he was the Pakistani. But I was conceived as a result of a one-night-stand; he possibly didn't even know I was ever born. It would be almost impossible to trace him and I'm not sure I would gain anything anyway. My mother carried me and bore me; he just had a good time for one night. Also, she's told me that he looked exactly like me, which answered my original question."

Shaun says he's not sure how much he would be able to give to a relationship with his birth father. "Would the bond that might have been there if he'd have known about me all these years even exist? I'm not so sure, so what would be the point in searching? I have a lovely adoptive family and even though they know I've searched and found my birth mother, I'm not sure I'd want to rock the boat by going down the route of finding my birth father too. I think my dad, who I'm very close to, might feel some resentment."

Pam Hodgkins, chief executive of Norcap, the charity for adults affected by adoption, says that in the majority of cases of adoptees searching out their roots, the starting point is the birth mother. "When people make the initial inquiry, they often claim not to be interested in the birth father." Over time, though, adoptees often become more inquisitive. "Generally, it's about completing the jigsaw, and for those who have no hope of finding out about him, it can be devastating. That said, other people do accept it, although this is more likely when they have a positive relationship with the birth mother."

For Lucy Warren, 61, the limited knowledge she has about her father has been enough to put her off wanting to know any more. "I was born just before the end of the war to an unmarried 18-year-old and was brought up by my grandmother. When I was nine, I was told that my father was a pilot, who had been unable to marry my mother because he had to return to his life in Canada as a mountie. I accepted that and even vaguely toyed with the idea of searching for him. It was only eight years ago, after my mother died, that a family member told me the true story: my father was a volunteer orderly who got my mother pregnant, then got his friend to pretend to be a vicar and perform a wedding ceremony. Off she went back to her family to say she was going to be a GI bride and live in Canada, only to discover that he'd left the country. I decided at that point that I never wanted anything to do with him.

"And in a funny sort of way, making the decision not to seek him out gives me back a feeling of control," she adds.

Perhaps most infuriating of all are those situations where people know that someone else holds the key to information about their paternity. "Because most father's names aren't on adopted people's birth certificates, they rely on information in their adoption records, but some adoption agencies may still not provide this information, so the person is then left with a complete blank," says Julia Feast, policy research consultant at the British Association for Adoption and Fostering. Given that her own research recently found that more than 80% of people who accessed information about their birth records said it really helped their wellbeing, she believes this practice is unfair.

Meanwhile, for Tracy-Ann Williams, 32, it's her mother who she believes holds the key. "I was raised by my mother and grandparents and never had a father. It was never mentioned, and when I asked about it in my early teens, I remember being told I was too young to know. Later, at about 17, my mother said it was a brief fling on a skiing holiday and that she can't remember his name, but I'm not convinced. I've had one-night stands and not had a baby, and I remember their names. Also, she just isn't the type for one-night stands. The last time I mentioned it was in my mid-20s and she got upset. I would love to look for him and I feel angry that my mother isn't telling me everything, or at least I don't think she is."

Some of us are oblivious to the fact that we don't know our true paternity. In the 70s, one study accidentally discovered that up to 30% of a group of some 250 women had a child who could not have been the offspring of its putative father. And it still remains the case that the mother's partner (not the real father) is named on the birth certificate of someone who is donor-conceived. "This means that many, many people still don't know they are donor-conceived," says Olivia Montuschi, founder of the Donor Conception Network (although, interestingly, one study found that many people who eventually discovered they were donor-conceived had realised something didn't quite fit). Montuschi is now involved in persuading parents to tell their donor-conceived children from as young as possible. "Thankfully, anonymity is no longer an option for donors, but we still need to encourage parents to tell in the first place."

In other situations, including adoption, the abiding rule is also that it is never, ever too early for a child to be told the truth in an age-appropriate way. "People underestimate what children can cope with and the longer you put it off, the more difficult it can be," says Dr Pettle. Even where there is little or no information to pass on, it's generally accepted that a child should grow up knowing that.

Yvonne Gibson, legal adviser on the Buckinghamshire adoption panel, says: "There is much more emphasis now on the child's right to family life under the Human Rights Act and thus their right to know both parents. In the past it was largely up to the mother to decide if the father was told about the child's birth. Now, unless there are exceptional circumstances, the father would be contacted."

Nevertheless, Stella believes we still have a long way to go. "A lot of people think they have a right to have children, regardless of whether the child has any information about their father. It's within this context that you still get people fighting for anonymity among sperm donors which, in light of experiences like mine, seems incredible."